1. Technical Field of the Invention
This invention relates generally to digital communications; and more particularly to higher order modulations used in high-speed digital communication systems.
2. Description of Related Art
In digital communication systems, more than one digital modulation scheme may be employed for the transfer of digital information. Digital modulations are generally susceptible to noise and other signal impairments introduced by the communication channel. In order to cope with different channel conditions, some high-speed digital communication systems use a multitude of digital modulations, e.g., QPSK, 8-QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM, etc., and different levels of coding and transmit power. While one channel may support a lower order modulation, e.g., QPSK, permitting only a lower data rate, another channel may support a higher order modulation, e.g., 64-QAM, providing higher data rate. Commonly used coding schemes include forward error correction (FEC) coding such as Reed-Solomon coding and inner channel coding such as convolutional coding, trellis coded modulation, or Turbo Coding, etc. Between FEC coding and inner coding the order of data is usually permuted by an interleaver.
Multiple digital modulations are employed in a variety of digital communication systems, e.g., in voiceband modems, digital subscriber links (DSLs), cable modem systems, and a wide variety of wireless systems. The particular manner in which higher order constellations are constructed and the way in which coding is employed and bits are mapped into modulation symbols represents an important topic in designing communication systems.
Sometimes there exists a need to augment existing standards for communication systems with additional capabilities. For example, in the particular case of downstream transmission in cable modem systems one may want to add to existing 64 QAM and 256 QAM modes, for example.